The 5-HT(3) subtype of a serotonin receptor is unique in that it is directly coupled to an ion channel and functions in rapid synaptic transmission in the brain. The functional properties of this receptor are important for understanding the role that serotonin plays in brain functions. One aspect of the 5-HT(3) receptor investigated in the lab is the phenomenon of desensitization, whereby the 5-HT(3) receptor channel opens and then closes in the continued presence of 5-HT. Many factors regulate the rate of desensitization, including the primary amino acid structure of the receptor channel, membrane voltage, calcium (both intracellular and extracellular) and the phosphorylation state of the channel. I am currently studying how these and possibly other factors control the rate of desensitation of the 5-HT(3) receptors expressed in Xenopus oocytes using voltage-clamp techniques, as well as native 5-HT(3) receptors expressed in the NG108-15 neuroblastoma cell line. Intracellular calcium also profoundly effects the function of the 5-HT(3) channel; interestingly, chelating internal calcium greatly reduces the rate of desensitization for expressed channel in Xenopus oocytes and increases the rate of desensitization for native channels in NG108-15 cells. Signal transduction pathways are thought to be involved in both cases, the molecular details of which are currently being explored. During the past year, work appeared showing that this channel is not permeable to calcium. Lastly we have evidence that casein kinase II (CKII) appears to decrease the rate of desensitization of 5-HT(3) receptors in NG108-15 cells. As there are two developmentally regulated forms of this channel that differ in a putative intracellular consensus site for CKII, this has profound ramifications for a putative developmental role for this enzyme in regulating the functional properties of the 5-HT(3) receptor channel. The aim of this work is to determine precisely what parameters affect channel opening, and its modulation.